Greener gym conserving power (6)

I believe the answer is:

energy

I'm a little stuck... Click here to teach me more about this clue!

'power' is the definition.
(synonyms)

'greener gym conserving' is the wordplay.
'ENERGY' is hidden within the letters.
But, I'm unsure how this is indicated.

Can you help me to learn more?

(Other definitions for energy that I've seen before include "Seventeen forty six defeat for Bonnie Prince Charlie" , "Pep" , "'Vigour, vitality (6)'" , "Vigour, vim, vitality" , "Verve" .)

I've seen this clue in The Guardian.

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